1. Field of the Invention
The present invention generally relates to image pickup apparatus and, more particularly, to an exposure control apparatus for use with a solid state imager such as a charge-coupled device (CCD) imager or the like.
2. Description of the Prior Art
In a conventional video camera or the like, an auto iris mechanism in which a so-called iris (i.e., stop) housed in a lens is automatically adjusted is known as a mechanism which automatically adjusts a quantity of light received by a so-called CCD (charge-coupled device) sensor, that is, a solid state imager formed of a photo-electric conversion element, the CCD or the like. Japanese Laid-Open Patent Publication No. 63-82067, for example, describes the mechanism which will hereinafter be referred to as an automatic exposure control mechanism.
More specifically, as shown in FIG. 1, this conventional auto-iris mechanism is generally composed of a lens unit 50 and a video camera body 60.
As shown in FIG. 1, the lens unit 50 is composed of a lens 51, an iris 52, a level detecting circuit 53 for detecting the level of an imager signal supplied from the video camera body 60, a comparing circuit 54 for comparing an output of the detecting circuit 53 with a reference voltage and an iris driving circuit 55 for controlling open and/or close of the iris 52 on the basis of a compared output of the comparing circuit 54.
The video camera body 60 is composed of a CCD image sensor (hereinafter simply referred to as a CCD) 61 which might be a solid state imager, an amplifying circuit 62 for amplifying an imager signal from the CCD 61, an automatic gain control (AGC) circuit 63 for automatically controlling the gain of the imager signal thus amplified by the amplifying circuit 62, and a signal processing circuit 64 for converting the imager signal from the AGC circuit 63 into a video signal conforming to the so-called NTSC or PAL system and so on. The video signal thus converted is developed at a terminal 65.
The auto-iris mechanism achieves its function by feeding the output level of the CCD 61 housed in the video camera body 60 back to the iris 52 housed in the lens unit 50. That is, the iris 52 is automatically controlled to open and/or close in such a fashion that the output level obtained from the CCD 61 through the amplifying circuit 62 and the detecting circuit 53 becomes coincident with the reference voltage, that is, the output of the comparing circuit 54 becomes zero.
Further, as an exposure control mechanism which does not utilizes an iris, the assignee of the present application has previously proposed an electronic shutter mechanism in which, for example, a charge storage time of a CCD image sensor of a so-called field storage type is controlled. This electronic shutter mechanism is described in our co-pending Japanese Patent Application No. 2-238930 (see also U.S. patent application Ser. No. 07/755526).
To be more concrete, in the CCD image sensor of the field storage type having the electronic shutter function, an image read-out pulse SG (high level) shown in FIG. 2B is supplied thereto in response to a signal VBLK of low level indicative of a so-called vertical blanking period (hereinafter, referred to as a vertical blanking signal) shown in FIG. 2A, and electric charges stored during the period from the supply of the image read-out pulse SG of an arbitrary field to the image read-out pulse SG of the next field are read out on the basis of the image read-out pulse SG of the next field.
According to the electronic shutter function, as shown in FIG. 2C, after the image read-out pulse SG of an arbitrary field is supplied, a high level pulse (hereinafter referred to as a reset pulse) SUB is supplied to a so-called substrate of the CCD image sensor during a so-called horizontal blanking period to discharge electric charges stored up to that time to thereby control a time period from the supply of the last reset pulse SUB to the supply of the image read-out pulse SG of the next field, thus an electric charge storage time T.sub.CHG being controlled. For example, in the NTSC system, the maximum electric charge storage time T.sub.CHG is 16.7 milliseconds determined by the field frequency, whereas in the PAL system the maximum electric charge storage time T.sub.CHG is 20 milliseconds determined by the field frequency.
An industrial video camera, for example, utilizes many interchangeable lenses based on a so-called C mount system and lenses of the video camera body can be freely combined in use. However, the auto-iris lens employing the above-mentioned auto iris mechanism has various problems of the interconnection (interface) with the video camera body. For example, there are disadvantages such as compatibility of a connector for connecting the auto iris lens and the video camera body, the matching of standards such as power source voltage supplied to the auto iris lens from the video camera body, current capacity, level of feedback signal or the like.
Further, as shown in FIG. 1, the detecting circuit 53, the comparing circuit 54 and so on are housed within the lens unit 50 so that, each time the lens unit 50 is exchanged, then the reference voltage or the like must be adjusted in the lens unit 50 so as to obtain an optimum exposure.
Further, the auto-iris lens is expensive as compared with a manual iris lens whose iris must be adjusted in a manual fashion and the interconnection work via the cable is cumbersome.
Furthermore, in the exposure time control done by the application of the electronic shutter function, the reset pulse SUB for discharging stored electric charges must be supplied during the horizontal blanking period so as not to affect the image signal now read-out. As a consequence, as shown in FIG. 2E, the electric charge storage time T.sub.CHG is controlled in the unit of time corresponding to one period of a so-called horizontal synchronizing signal (hereinafter referred to as a 1H), that is, in the unit of 64 microseconds. Accordingly, in a low speed shutter region in which an object is dark and a shutter speed is slow, the electric charge storage time T.sub.CHG can be controlled stepwise without causing any trouble. Conversely, in the high speed shutter region in which the object is bright and the shutter speed is fast, the step width is so coarse that this method is not suitable in practice.
In this case, the highest shutter speed which can be controlled by the conventional electronic shutter is about 1/10000 second so that, if the shutter speed is increased more, then the incident light amount cannot be satisfactorily controlled when the cameraman takes a picture in actual practice.
As described above, only by controlling the shutter speed, it is difficult to adjust sensitivity upon taking a picture.